Four Intracisternal Calcium-binding Glycoproteins from Rat Liver Microsomes with High Affinity for Calcium

Abstract

It has been claimed that the inositol 1,4,5-trisphosphate-sensitive calcium pool in liver and pancreatic acinar cells is located in specified organelles ("calciosomes") which are characterized by their content of the calcium-binding protein calsequestrin (Volpe, P., Krause, K. H., Hashimoto, S., Zorzato, F., Pozzan, T., Meldolesi, J., and Lew, D. P. (1988) Proc. Natl. Acad. Sci. U. S. A. 85, 1091-1095). We show here that the inositol 1,4,5-trisphosphate-sensitive compartment of rat liver does not contain calsequestrin-like material. Instead four non-membraneous calcium-binding glycoproteins with approximate molecular masses of 59, 60, 80, and 90 kDa were found. The 59-, 80-, and 90-kDa proteins were of the high mannose-rich type, the carbohydrate moiety of the 60-kDa protein was of the complex hybrid type with terminal galactoses. All four proteins had high affinity binding sites for calcium (KD between 1 and 5 microM) and from 1 to 5 binding sites/molecule. The 80- and the 90-kDa proteins had also low affinity binding sites (KD 400 and 600 microM, respectively, with 13 and 15 binding sites/molecule, respectively). A comparison of the NH2-terminal sequences revealed that the 60-kDa calcium-binding protein represents the rat liver calregulin, whereas the 90-kDa calcium-binding protein represents grp94. The sequences did not reveal any relationship of the 80-kDa protein with grp78, or of the 59-kDa protein with protein disulfide isomerase.